The function of transfer assist blades is generally for pressing a copy substrate into intimate contact with the toner particles on a selectively charged imaging surface, such as a photoreceptor, during image transfer from the charged imaging surface onto the copy substrate. In particular, non-flat or uneven image support substrates, such as copy sheets that have been mishandled, paper that has been left exposed to the environment, or substrates that have previously passed through a fixing operation (for example, heat and/or pressure fusing) often tend to yield imperfect contact with the photoconductive surface. Some printing applications require imaging onto high quality papers having surface textures which prevent intimate contact of the paper with the developed toner images. In duplex printing systems, even initially flat paper can become cockled or wrinkled as a result of paper transport and/or the first side fusing step. Also, color images can contain areas in which intimate contact of toner with paper during the transfer step is prevented due to adjacent areas of high toner pile heights.
The lack of uniform intimate contact between the imaging surface and the copy sheet in these situations can result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface and the copy substrate. When spaces or gaps exist between the developed image and the copy substrate, various problems may result. For example, there is a tendency for toner not to transfer across gaps, causing variable transfer efficiency and, under extreme circumstances, creating areas of low toner transfer or even no transfer, resulting in a phenomenon known as image transfer deletion.
In order to minimize transfer deletions, transfer assist blades (TABs) have been utilized to press the back of the copy substrate against the imaged area of the charged imaging surface. The transfer assist blade is typically moved from a non-operative position spaced from the copy substrate, to an operative position in contact with the copy substrate. A mechanism supporting the TAB is operable to press the TAB against the copy sheet with a typically pre-determined force sufficient to press the copy substrate into contact with the developed image on the photoconductive or other charged imaging surface in order to substantially eliminate any spaces therebetween during the transfer process.
For a number of reasons, no portion of the transfer assist blade should contact the imaging surface. Such contact may result in the pick up of residual dirt and toner from the charged imaging surface onto the portion of the transfer assist blade that contacts the imaging surface. More significantly, contact of the TAB with the charged imaging surface risks abrading the surface, thereby adversely affecting subsequent image quality and shortening the expected life of the expensive photoreceptor or other charged imaging surface.
In order to ensure that a transfer assist blade does not contact the imaging surface beyond the sides of the copy substrate perimeter, either the transfer assist blade is shortened to correspond to the narrowest copy sheet width expected to be processed in the printer, or the effective length of the transfer assist blade is varied to correspond to the width of the substrate. An apparatus such as that disclosed in U.S. Pat. No. 6,687,480, issued to Obrien et al., is capable of varying the effective length of the transfer assist blade to account for different substrate widths.
As explained above, it is important that the TAB be raised and lowered so as not to contact the photoreceptor or other charged imaging surface when the substrate is not in contact with the TAB. As a counterpoint, it is also important that the TAB contact the back of the copy substrate as close as possible to the leading and trailing edges of the copy substrate in order to ensure contact in all imaging areas. A high degree of accuracy is therefore required in timing engagement and disengagement of the TAB with the copy substrate. Such engagements and disengagements of the TAB are generally designed as timed sequences in relation to paper path speed and the sensed width in the paper path of the copy substrate. As an example, U.S. Pat. No. 6,556,805, issued to Kuo et al., teaches a method of activating TAB segments by rotating one or more cam shafts, thereby pressing the TAB into contact with the copy substrate when the appropriate cam lobe has been rotated. Another system for activating TAB motions is taught in U.S. Pat. No. 6,188,863, issued to Gross et al. Any number of other systems have been utilized and many more are possible.
In the typical cam system, there is a timing delay between commencement of rotation by the cam shaft and contact between the TAB and the copy substrate. Similarly, there is a timing delay between sensing of the leading or trailing edge of a copy substrate and actuation or deactivation of the cam shaft rotation or other mechanism that urges the TAB toward the copy substrate. Such timing sequences are typically handled during machine design and initial system calibration. Conventionally, the calibration is performed manually by such means as attaching an ink pad to the blade, measuring the length of the mark that the pad makes on the back of a copy sheet, and calculating the required adjustment time to achieve the desired length of such mark.
As printing system speeds increase, the speed of the copy substrate along the paper path increases, and TAB activation and deactivation must be timed more perfectly to ensure proper placing of the TAB as close as possible to the leading and trailing edges. Moreover, initial calibrations of the timing sequence may be obsoleted as components affecting the sequence are replaced over time with replacement components that vary slightly in response time, size, shape, etc. In particular, a replacement TAB can vary slightly in length, thickness, position within its mounting, and each of these factors may affect the timing of TAB contact with a copy substrate. Additionally, normal wear and tear and “settling in” of cams, motors, gears, photoreceptor belts, and other components can affect the precise timing sequence of TAB actuation apparatus. Additional calibrations are possible but typically require the time, expense, and labor of service and maintenance calls. It is advantageous for electrostatographic imaging systems utilizing TAB-type devices to have a timing adjustment system wherein the timing of TAB activation and deactivation is adjusted to account for any of the changes that may affect the TAB timing sequence.
In one such system, disclosed in U.S. Pat. No. 6,485,224 issued to Gross et al., there is provided an apparatus for adjusting the timing of contact between a transfer assist blade and a charged imaging surface in order that the timing be automatically adjusted within specifications. The apparatus comprises an imaging apparatus for developing a partially toned pattern having about 20 to about 80 percent coverage in a region of a charged imaging surface; a transfer assist blade movable between a position engaged with a surface and a position disengaged from the surface, and a drive device for imparting engagement and disengagement motion to the transfer assist blade. The drive device has an activation time for engaging the transfer assist blade with the surface and a deactivation time for disengaging the transfer assist blade from the surface. A toner area coverage measuring device measures the percentage of the partially toned region that is covered by toner and feeds data to a controller for adjusting the timing of activation of the drive device. In particular, the apparatus in the '224 patent utilizes the toner area coverage measuring device to determine whether the time of activation has resulted in engagement of the transfer assist blade outside of the specifications. If so, the controller automatically adjusts the timing of activation accordingly. The disclosure of the '224 patent is incorporated herein by reference, and especially the description of the TAB drive device and the controller.
In response to the needs left unmet by these prior systems, an apparatus is provided for assisting in the transfer of an image from an image bearing member onto a copy substrate that comprises an image bearing member carrying an image to be transferred, a feed mechanism for feeding a copy substrate to the image bearing member, and a transfer assist mechanism movable to an activated position bearing on the substrate to maintain the substrate in contact with the image bearing member to assist in transferring the image thereto, and to a de-activated position. A sensor generates a signal in response to passage of the leading edge and the trailing edge of the substrate to ascertain an actual length of the substrate as it moves to the image bearing member. A controller is operable to direct the transfer assist mechanism to move to the activated position and then to direct the transfer assist mechanism to move to the de-activated position after a de-activation dwell time. The de-activation dwell time is at least initially a nominal dwell time based on an expected length of the substrate from its leading edge to its trailing edge. The controller is operable to change the de-activation dwell time from the nominal dwell time in response to the determination of the actual length of the substrate.
A method is further provided for operating a transfer assist mechanism to bear on a substrate passing over an image bearing member for transferring an image onto the substrate. The method comprises activating the transfer assist mechanism to bear on the substrate, determining a nominal dwell time for de-activation of the transfer assist mechanism as a function of an expected length of the substrate, sensing an actual length of the substrate as it is conveyed to the image bearing member, and changing the dwell time for de-activation as a function of a comparison between the actual length and the expected length.